Use UUIDs in /boot/grub/grub.cfg for VM builds rather than hardcoding /dev/sda

This allows booting the image in KVM as either sda or vda without requiring any manual changes in /boot/grub/grub.cfg.

Use `FIXED_DISK_IDENTIFIERS="yes"` for VM builds by default.

This branch is based on my configurable_disk_identifier branch (#28), so please review and merge that one first so this one becomes mergeable.

Please review and merge.